Method of continuously casting tubes using a rotating mandrel



NOV. 5, 1968 c, YEARLEY ET AL 3,409,068

METHOD OF CONTINUOUSLY CASTING TUBES USING A ROTATING MANDREL FiledMarch 1, 1966 INVENTORS. DOUGLAS C. YEA/H53 HARRY h. $7007:

United States Patent O METHOD OF CONTINUOUSLY CASTING TUBES USING AROTATING MANDREL Douglas C. Yearley, Westfield, and Harry H. Stout, Jr.,

Plainfield, N..I., assignors to Phelps Dodge Copper ProductsCorporation, New York, N.Y., a corporation of DelawareContinuation-impart of application Ser. No. 468,771, July 1, 1965. Thisapplication Mar. 1, 1966, Ser. No. 530,869

4 Claims. (Cl. 164-85) ABSTRACT OF THE DISCLOSURE Tubing is continuouslycast by solidifying molten mate rial as it is withdrawn from a castingzone defined by a rotating central mandrel and a surrounding mold.

DISCLOSURE OF INVENTION This application is a continuation-in-part of anapplication by Douglas C. Yearley, Ser. No. 468,771 filed July Thisinvention relates to a method and apparatus for the continuous castingof hollow metal tubes. It has particular reference to apparatus for suchcasting which includes a rotatable mandrel and to the related method.

Continuous casting of tubes comprises the uninterrupted flow of moltenmetal to one end of a casting mold, uninterrupted withdrawal of solidmetal from the other end of the casting mold and the removal of heatfrom the metal to solidify it as it is formed into the desired sectionalshape during passage of the metal through the mold. Tubing is castcontinuously by the use of an inner core or mandrel positioned within acylindrical mold or die. The molten metal is supplied under pressure orby gravity feed to the annular space between the mandrel and thecylindrical mold. The mold is cooled to solidify the metal, and casttube is drawn from the mold by rollers or the like which grip the outersurfaces of the tubing.

In general, the relatively slow rates of continuously casting tubing inprior apparatus and the poor quality of such tubing are serious problemswhich heretofore have made it economically impractical to continuouslycast tubing for direct fabrication into commercial tubing. Thoseproblems are due in part to adhesion of the metal to the surfaces of themold and the mandrel and to shrinking of the metal on the mandrel duringthe cooling and solidifying phase of the process. These diflicultieshave persisted despite various counteracting measures which have beenattempted, such as vibrating the mold and providing the mandrel with ataper in the direction of the metal movement through the mold.

Other factors which have precluded extensive use of continuous tubecasting for direct fabrication to commercial tubing include thedifiiculty in starting the tube casting process, the tendency for thecast tube to have an intolerable degree of eccentricity and lack ofcontrol over the solidification process in the mold during the castingoperation.

Control over the solidification process is important since the processof solidification, including the rate of solidification and the positionand stability of the solid/liquid interface, determines the propertiesof the resulting tubing such as surface quality, tensile strength,elongation, grain size, etc.; and lack of control of that process will,of course, give poor properties in the tubing.

This invention provides an improved apparatus for the continuous castingof tubes and a new method for such casting, which will allow for themanufacture of high quality tubes on a commercial and economical basis.It eliminates start-up difficulties, provides a concentric tubing ofgood quality which may be rapidly and economically fabricated intocommercial tubing. Another important aspect of this invention is that itpermits the molten metal used to form the tubing to be fed to thecasting mold by gravity without any need for metering that flow or theuse of a feed spout to deliver and position the metal in the mold.

According to the present invention, the mandrel is mounted for verticalmovement relative to a mold which is fed from a molten pool of metalmaintained in a reservoir above the mold. The mandrel is adapted topermit rotation relative to the mold. In starting the tube castingoperation, molten metal is first fed to the mold from the overlyingreservoir while the mandrel is in a withdrawn position whereby solid rodis drawn initially from the exit or lower end of the mold; and then, asthe feed of molten metal is continued, the mandrel is lowered throughthe reservoir so that the tapered lower end of the mandrel enters themold to a depth sufficient to effect transition from rod to tubecasting. The mandrel is guided in its vertical movement by guide meansadjustable laterally to effect accurate centering of the mandrelrelative to the mold.

According to our method, rotation of the mandrel with respect to themold, either in a clockwise or counterclockwise direction or withrotational oscillation, has been found to improve substantially themetal grain and thus the ductility of the inner surface of cast tubes.We believe that any such mandrel rotation, within limits as to therotational rates, will improve the ductility of the inner surface of atube cast according to our method. When castings are made without anyform of mandrel rotation, the inner surface of the tube lacks ductilityand may craok when subjected to plastic elongation (i.e. an internalbend test or a standard tensile test). These cracks also develop as lapson the internal diameter of the tubing during cold drawing, causing theresultant finished tube to be rejected as defective. When tubes are madewith a rotating mandrel, no cracking occurs in the internal diameterwhen elongated and the casting can be successfully fabricated.

The direction of rotation of the mandrel according to -our method may beclockwise or counterclockwise or it may be oscillatory wherein thedirection is reversed periodically. Such reversal may occur after lessthan a complete revolution, at the end of each revolution, or at the endof several revolutions. Satisfactory castings have been obtained over awide range of rotational speeds up to 900 rpm. The preferred rangeappears to be between 10 and rpm.

As stated, mandrel rotation according to our method will improve theductility of the inner surface of the cast tubing. However, the mannerof rotation of the mandrel may induce or reinforce an adverse swirleffect upon the outer surface of the tubing, which also must beconsidered to determine the optimum manner of rotation of the mandrel.

While mandrel rotation above a minimum rate and not at excessive rateswill improve the ductility of the inner surface of a tube cast accordingto our method, if the rotation of the mandrel reinforces the swirl whichwould otherwise exist, cracking may occur in the outer surface of thetube. Mandrel rotation which minimizes the swirl is therefore generallypreferred.

if wh a li d .draifis .th QQ h n p i u e thewalltube, particularly at ahigh casting rate. In such a" case it is difficult exactly to counteractthe swirlwhich would otherwise exist because' an undesirable counterswirl is readily induced by mandrel rotation. We have found that in sucha case it is preferable to oscillate the mandrel and that the spiraleffect is substantially eliminated. When casting thick wall tubes it issufiicient to rotate the mandrel in the direction counter to that of theswirl which would exist under the casting conditions if the mandrel werenot rotating.

The thinner the wall thickness of the casting made according to ourmethod, the greater the tendency for the metal to swirl in the dieincreasing the spiral effect and causing the resulting cast tube tocrack when elongated. The faster the casting speed the greater thetendency for such cracking. In general, when casting-a thin wall tube weprefer to use a slower rotational speed for the mandrel and oscillatorymotion of the mandrel in such a case is preferred. In casting a thickerwall, we, therefore, prefer to use a relatively higher mandrelrotational speed than for tubes having thinner walls, and for thethicker wall tubes, in general, we prefer to rotate the mandrel in onedirection, counter to the direction of swirl which would exist under theconditions of casting with the mandrel not rotating. 'By a thin walltube we mean one having a wall thickness on the order of 0.2 of an inchand by a thick wall tube we mean one having a wall thickness of theorder of 0.5 of an inch.

Using our invention, 3 inch diameter tubes having a wall thickness of.380 have been successfully cast from phosphorus bearing copper at 20inches per minute at mandrel rotation speeds between 20 and 40 rpm. ineither direction and at 20 r.p.m. elfective speed with the directionreversed every revolution to provide oscillatory motion. Without mandrelrotation, cracks would result in the internal surface of such a tube ifit were elongated.

The method and apparatus of our invention may also be used successfullyto produce good quality tubes (for example, made with phosphorusdeoxidized copper) hav- I ing overall diameters of 1 /2 to 4 inches withwall thicknesses ranging from 0.17 to 0.45 inch, at speeds in excess of2 feet per minute and with an error of concentricity of less than 0.01inch,

The invention may be better understood from the following description ofa specific example as illustrated by the accompanying figure, which is avertical view of a preferred form of the apparatus of our invention.

Referring to the figure, metal M in molten form is introduced from anexternal source, no shown, by means of a launder or trough 11 into agenerally cylindrical crucible 12 of graphite mounted on a verticallymovable platform 13. The latter has a generally rectangular shape inplan view and any conventional means (not shown) may be used to supportit. i

The platform 13 has attached to it intermediate its right front and rearcorners and left front and rear corners support 36 and 36',respectively. Struts 37 and 37' reinforce cross bracket 35 which isattached at each of its ends to supports 36 and 36'. These memberssupport the upper portion of the apparatus, as more fully hereinafterexplained. v v

Heat from crucible 12 is insulated from platform 13 by insulating bricks16. The crucible is enclosed inan induction coil 17 carrying water asthe coolant and supas silicaor the like which is rammed between the coiland the outer surface of the crucible. The open end of the crucible issuitably provided with a plate 19 of ring form. Mounted within thecrucible 12 through a central aperture in the bottom thereof is a guidebearing 20 also of graphite and having generally radial and sloped feedports 21 to allow passage of the liquid metal into a cylindrical castingmold 22. The latter is closely surrounded by the lower portion ofbearing 20, and the upper portion of this bearingforms aclose slidingfit around a graphite mandrel 23v so as to center the mandrel in thegraphite casting mold 22.

The mandrel 23 is supported from an overhead support 25 from whichdepends (shown schematically) hoist means 26 for raising and loweringthe mandrel. The hoist 26 may be hand or motor driven, but preferably isa conventional hand operated worm gear which allows precise positioningof the mandrel. Suitably attached to hoist 26 is a depending eye bolt 27connected to one end of a strain measuring device 28. Elongation ofdevice 28, by tensile strain on the depending mandrel 23, is detected bystrain gauges 29 suitably mounted on strain device 28 and connectedelectrically to a conventional indicator (not shown) for indicating theamount of strain on the device 28 and thus the downward force exerted onmandrel 23 during the tube casting operation. As such strain measuringdevices are well known in the art, further description of the device 28and its associated elements is unnecessary.

The mandrel 23 is caused to rotate or oscillate by means of rotatingdevice 60. A variable speed motor 61 is connected to a right angle geardrive 62 by a shaft 69 having universal joints 66 and 67 and slidingcoupling 68. The vertical output shaft 65 of the right angle drive 62 isattached to' guide post 31 by coupling flanges 63 and 64 which is fixedto guide post 31 with bolt 30.

The lower end of mandrel rotating device 60 is connected by a bolt to aguide post 31. The latter is held in vertical alignment with the guidebearing 20 and mold 22 by a steel guide bearing 32 into which precisionmachined bronze bushings 33 have been pressurized. Guide bearing 32 ismounted in an alignment box 34 secured to cross bracket 35. Orientationof the guide bearing 32 in g the mounting box 34 is adjustable by bolts38 for angular positions with the vertical and is adjustablehorizontally by radial bolts such as bolt 39. Guide bearing 32 is lockedin its adjusted position by a cover plate 40 and bolts 41.

Mandrel 23 is rigidly connected to guide post 31 by a connecting pin 43.A cross bolt 44 connects one end of the connecting pin 43 to the guidepost 31 while a similar cross bolt 45 connects the mandrel 23 to theother end of connecting pin 43. To prevent wobble between the guide post31 and the mandrel 23, two lock-nuts 46 and 47 are threaded on pin 43and tightened against the guide post 31 and mandrel 23, respectively. 7

Liquid metal in the casting mold 22 is solidified in the mold portionlying within a cooling jacket 50. The

latter is provided at the bottom with an annular flange plate 52 throughwhich extend a plurality of studs 51 depending from platform 13. Jacketis held in position by nuts 53 threaded on studs 51 and bearing againstthe.

bottom of flange plate 52.

The metal solidifying between mold 22 and mandrel 23 is withdrawn astube 54, which passes downward into a quench chamber 55. Water passingthrough jacket 50, along the path indicated by the arrows of the figure,flows downward to chamber 55 to serve as the quenching liquid. Thequench chamber 55 is drained through exit port 56 to a recirculatingwater system with means to coolthe water (not shown), the cool waterbeing circulated to jacket 50 through inlet port 75. The cast tube isdischarged through the bottom of quench tank 55 by way of a sealedoutlet 57 welded to the quench tank 55. The tube is pulled downwardlythrough seal 57 by withdrawal rolls (not shown). 7

Graphite guide bearing 20 is centrally mounted in crucible 12 by apress-taper fit. A lining sleeve is provided on the inner surface of thegraphite guide bearing 20 and may be easily replaced in the event ofwear to insure a close-tolerance slide fit between the mandrel 23 andguide "bearing 20. Mandrel 23 is positioned in mold 22 by operation ofhoist 26 and adjustment of box 34 (which may be adjusted upon wearing ofgraphite guide bearing 20), to effect concentric solidification. Thus,the cast tube 54 will have essentially a uniform wall thickness.

Mandrel 23 is cylindrical for substantially its entire length from itsupper end to a distance just short of its lower end. This cylindricalportion is indicated by numeral 231. The next lower portion 232 is givena slight taper and the next lower portion 233 has a greater taper ofabout twice the rate of the taper of portion 232. The tip portion 234 isnot necessarily tapered, although it is preferable to provide it with apoint to allow the mandrel to penetrate easily the surface of the moltenmetal as the mandrel is lowered into the mold. In one design for amachine casting a tube with a 3-inch O.D., the mandrel was 2 feet long,portion 231 was 20 inches long with a diameter of 2.35 inches, portion232 was 2.5 inches long with a taper of 1 while portion 233 was about1.5 inches long and had a taper of 2. The taper of each of the twoportions is not critical, however, since the mandrel can be positionedduring the casting to the precise position required to satisfy therequirements of the tube being cast. It is also satisfactory to use amandrel having a single tapered portion having a taper on the order ofabout 1 and being as long as the effective length of the metalsolidification zone, which is generally in the range of about 3 to 9inches.

The casting mold assembly includes the graphite liner 22 which is placedwithin the cooling jacket 50 and firmly seated and sealed against waterleaks. Water under pressure is fed through a flexible hose (note shown)to inlet nipple 75 of cooling jacket 50, rising vertically and enteringa narrow annular gap between the graphite die and an inter-bafile 79.The water then flows downward at high velocity through the annular space80 within bafile' 79 and impinges on the surface of existing cast tube54.

Before the start of casting, the graphite crucible 12 is preheated, asby means of the induction heating coil 17, to temperatures in excess ofthe melting point of the metal or alloy to be cast. A moderate flow of acoolant, such as cold water, is circulated through jacket 50 to preventoverheating of the mold 22. The graphite mandrel 23 is fully withdrawnfrom the mold into the guide bearing 20, and the mandrel rotated. Themolten metal or alloy M is then introduced into the crucible 12 fromwhich it flows through the feed ports 21 in guide bearing 20 into mold22. A starting cup (not shown), which has previously been placed in themold, receives and holds the liquid metal until it solidifies. Theresulting solid rod, serving only as a starting rod, is withdrawn; andthe coolant water flow rate is increased sufficiently to keep thetemperature from rising while the solid rod is continuously cast fromthe mold. Casting speeds are then increased to the desired steady-statetube casting rate by increasing the speed of the withdrawal rollers.

The mandrel 23 is then gradually lowered into the mold to form thecontinuously cast product into a tube. This transition from rod to tubeis made with care to prevent a sudden grabbing of the mandrel by thesolidifying metal, causing a possible fracture of the cast. Thistransition may be facilitated by the use of easily detectable loadindications on the strain gauges 29 calibrated according to theparticular casting desired.

The zone within the mold, where the molten metal becomes solidified fortube casting, is displaced downwardly from the zone where solidificationfor casting the solid rod occurs. The position of this zone iscontrolled by the positioning of mandrel 23. If the mandrel is inserted(lowered) beyond the proper position, excessive shrinkage pressure isdeveloped on the mandrel causing transverse checks and cracks on theinside surface of the tube. If the mandrel is insufficiently lowered tothe proper position, ripples are produced on the inner surface of thetube.

As the tube is first cast, test cuts through the tube indicate deviationin concentricity of the mandrel with respect to the mold. Thesedeviations or error in concentricity can be corrected by adjustments,during the casting operation, of the lateral position of mandrel 23 orin the mold 22 as described above. If irregular surface conditions,particularly the internal surfaces, are detected the mandrel position iseasily adjusted in angular relation to the mold to correct the surfaceirregularities. The tapered mandrel permits easy control of the wallthickness of the tube during casting by varying the vertical position ofthe mandrel. It may be desirable to vibrate the mold by any of severalconventional means (not shown).

Example Using the apparatus shown in the figure, we cast a 3 /2 inchdiameter tubeof phosphorus deoxidized copper having a wall of 0.310 ofan inch. The mandrel was rotated for 15 seconds at 20 rpm. in onedirection followed by 15 seconds of rotation at the same speed in theopposite direction.

Swirling of the molten metal in the mold was substantially reduced andthe resulting tube was substantially improved in quality. The interiorsurface of the tube was ductile and did not crack when subjected toplastic elongation.

The present invention makes it possible to increase substantially thequality and production rate of tubes continuously cast. The amount ofmaterial required during initial start-up of the process is reduced. Thegain structure and surface characteristics of the cast tubing aresubstantially improved. It also permits satisfaction of a wide range ofconditions and the continuous casting of a variety of alloys whilemaintaining desirable grain structure and surface characteristics.

The use of a rotating mandrel improves the ductility of the internalsurface of the cast tube and thus produces a tube which resists internalcracking if the tube is subjected to plastic elongation.

We claim:

1. A method of continuously casting tubes which comprises the steps ofcontinuously feeding molten metal into one end of a casting zone whilecooling said zone and while discharging a solidified metal product inthe form of a rod from the opposite end of said zone, graduallyinserting a mandrel into said zone to form an annular space between saidend of the zone while continuing said feeding, cooling and dischargingthe product, thereby converting the discharging product from a rod to atube, and rotating said mandrel within said zone to control thecharacteristics of the discharging tube.

2. A method according to claim 1, wherein said mandrel is rotated in arange of speeds between 10 and r.p.m.

3. A method according to claim 1, wherein said mandrel rotation isoscillatory motion.

4. A method of continuously casting tubes which comprises the steps ofcontinuously feeding molten metal into one end of a casting zone whilecooling said zone and while discharging a solidified metal product inthe form of a tube from the opposite end of said zone, rotating amandrel positioned within said zone to form an annular space between theends of said zone, and adjusting the rotation and position of themandrel to control the char acteristics of the discharging tube, saidmandrel being rotated in a direction and in a range of speeds between

